A new study by the Mercer global consulting firm says the Angolan capital, Luanda, tops the list as the world's most expensive city for expatriates.

In its annual cost of living survey of more than 200 cities, Mercer considered factors such as housing costs, transportation and food in compiling its list of the 10 most expensive cities.

Several factors have driven up costs for foreigners working in the capital of oil-rich Angola, said Mercer spokesman Steven Nurney.

"The cost of high-quality imported goods is very high there and probably the most significant aspect of costs there is the fact that high-quality and secure, and I underscore the word 'secure,' rental accommodations are very expensive there."

The firm said renting a luxury two-bedroom apartment in Luanda costs an average of $6,500 a month.

Moscow is the second most city for expatriates, followed by Tokyo, which topped Mercer's list last year.

"The reasons are really almost exactly the same as the high ranking for Luanda. Again, the costs of imported goods and services is very high there and, again, the big driving factor there in terms of cost of living is suitable, secure high-quality rental costs, accommodation costs."

The remaining cities on the list are Singapore, Hong Kong, the Swiss cities of Geneva, Zurich and Bern and, finally, Sydney, Australia.

Researchers disagree on the most expensive city, overall. A February study conducted by the The Economist ranked Tokyo as the most expensive city. In a September 2012 survey, the UBS investment firm said Oslo, Norway, was the most expensive.